1. Field of the Invention
The present invention relates to a helical winding, of the type formed by a band or ribbon in a wound arrangement having a length considerably exceeding its diameter, the winding being adapted for implantation into a patient's body, as well as to a method of manufacturing such a winding of the type including the steps of winding the band or ribbon on a mandrel to obtain a winding of desired length and removing the mandrel from the wound winding, and to an implantable heart stimulator and a surgical tool employing such a winding.
2. Description of the Prior Art
A helical winding of the type described above is known from U.S. Pat. No. 5,423,881, which discloses a medical electrical lead having a conducting wire wound in a helical configuration. The lead is intended for use for endocardial stimulation by an implantable heart pacemaker and includes an electrode at the distal end of the lead and a connector at the proximal end.
This kind of winding has to be flexible to make introduction into e.g. a vein of a patient possible, and a problem with such flexible windings is that a specified length of the winding cannot be maintained during implantation and explantation. Especially in connection with explantation the winding is easily extended (uncoiled). Another disadvantage of this type of winding is a low torsion stiffness.
In U.S. Pat. No. 5,456,707 a pacing lead in the form of a helical winding is disclosed, the torsion characteristics of which are improved by providing an increased stiffness of an intermediate length of the lead. This increased stiffness is obtained by shrinking a tubing of Teflon.RTM. over the intermediate section of the lead winding, or by a suitable coating sprayed onto the winding to restrict its flexibility, or by a tube which is force fit between the conductor and inner diameter of the lead outer casing. This tube has a thickness so as to provide minimum clearance with the winding to prevent outward expansion of the winding, i.e. to increase the stiffness of the winding. These special measures for increasing the stiffness of the winding, however, complicate the manufacture of the lead and raises its price.
European Application 0 092 798 shows a multi-pole coaxial circuit intended to be used as an electrode for electric stimulation of body tissue. The circuit has at least one conductor in the form of a helically wound metal ribbon for reducing the diameter of the lead and improving its flexibility. This construction still does not solve the problem of extension of the winding, especially in connection with explantation of the lead, nor the problem of poor torsion stiffness.